Fiber optics communication links are used in numerous applications and, in particular, are extensively utilized as the primary means of carrying telecommunications and internet traffic between, and increasingly within, concentrations of users. These optical networks and their required management become increasingly sophisticated as their reach and capabilities increases. Of particular importance is reliable, electronically-automated means for setting and reconfiguring the interconnections and transmissivity levels of these links at the optical level so the various optical streams may be properly groomed and routed. Although it is possible to achieve these functions by converting the optical energy to electrical signals, routing the electrical signals, and converting back to optical form, much higher performance can be achieved by directly manipulating the optical streams. This is referred to as “transparent” operation, since once the setting is configured, the optical signals obey the set routing regardless of the nature of the information they carry. The physical implementation of setting and reconfiguring these transparent interconnections is a primary domain of optical switches and variable attenuators.
Current optical technology for switching and variable attenuation can be divided into two major classifications. The first and currently dominant approach is mechanical, whereby the physical displacement of at least one element of the optical path within the device changes the coupling of the optical signals at one input port from one output port to another. Mechanical switches can provide highly efficient switching, with very little of the optical energy getting out of the channel (low insertion loss) and extremely little optical energy leaking into unselected configurations (low crosstalk). Mechanical switches, however, tend to require rather bulky packaging to help isolate the optical paths from unwanted disturbances, and, being an assembly of mechanical movements and bulk optical devices, are not directly integratable with the other waveguide devices used in optical network management. Also, being an assembly with moving parts and requiring to maintain tolerances usually below 1 μm, mechanical switches elicit heightened concerns for reliability issues. Recently, several approaches have been undertaken to miniaturize mechanical optical switches using the micromechanical structures realizable in MEMS technologies. This provides some promise for improvement of the basic mechanical reliability and the potential to place more switch elements in a single package. However, these are still just miniaturized versions of bulk switches and still have significant lengths of optical path outside of waveguides and hence require extraordinary isolation from mechanical disturbances. Furthermore, while these approaches employ processes that can make multiple switches during a single process step, assembly constraints for interfacing optical fiber or other guided-wave components to the free-space switch still limit the potential for mass production and there is no accommodation for integration with other waveguide elements.
The other major class of technology for optical switching and variable attenuation is refractive. Here the optical paths typically are wholly within waveguides and the distribution of refractive indices along the optical paths is altered by a stimulus, typically a local application of heat or electric field, to route the optical signals along selected branches of the waveguide network. Such switches and attenuators are directly integratable with other waveguide devices, and the production methods, including the necessary fiber bonding, are better suited for mass production. Since there is no physical displacement of the alignment for the optical paths, the solid-state switches promise attainment of improved reliability over mechanical switches, particularly as evaluated over large populations of switches in real-world deployments. These types of switches as standard components represent a less mature technology than standard mechanical switches. In order to achieve the desired sensitivity to the thermal or electric-field stimulus, the materials currently used for such waveguides do not provide lossless, polarization-insensitive transmission such as is obtained in high-quality silica planar waveguides. The constructed switches do not typically as yet provide as low insertion loss as mechanical switches and may not be capable of the ultra-high isolation that can be achieved in mechanical switches.
Another class of less widely pursued switching mechanisms that combine some of the characteristics of both mechanical and refractive approaches is based on microfluidics. Microfluidics encompasses a broad range of effects and the basic physics has been well studied for quite some time. Reported applications to optical switching are limited to one basic structure: a fluid region along the back of a turning facet at an abrupt waveguide bend or crossing [e.g., see J. L. Jackel et al., U.S. Pat. No. 4,988,157 (1991); M. Hideki et al., Jap. Pat. No. 6-175052 (1994); M. Sato, Jap. Pat. No. 7-092405 (1995); J. E. Fouquet et al., U.S. Pat. No. 5,699,462 (1997); D. K. Donald, U.S. Pat. No. 5,978,527 (1999)]. One fluid will have a refractive index near that of the waveguide mode, typically near 1.45, and when occupying the region behind the facet will render the reflectivity of the facet to be near zero. Optical signals encountering the facet in this condition will travel predominantly straight through the facet where they will couple into a coaxially-aligned waveguide just beyond the fluid region. When another fluid of much lower refractive index, typically a gas or vapor with index of refraction near 1.0, is moved to or created at the facet, the facet becomes nearly fully reflective. Optical signals encountering this facet will deflect off the facet and couple into another waveguide with an axis disposed at a position and angle with respect to the incoming waveguide so as to collect the deflected optical signal. In theory these devices can be very efficient. High efficiency requires well-aligned optical facets with very high optical quality; in particular, the reflective state poses stringent requirements on the quality of the facet because of the large refractive index contrast between the waveguides and the gas or vapor in the region behind the facet. In practice these facets are very difficult to make [e.g., see J. E. Fouquet et al., U.S. Pat. No. 5,960,131 (1999); J. E. Fouquet et al., U.S. Pat. No. 6,055,344 (2000); M. Sato et al., U.S. Pat. No. 6,072,9247 (2000)] and there are major challenges to overcome for making and assembling highly efficient devices with high yield. Further, most previously disclosed microfluidics-based devices rely on either a phase change or a substantial thermal gradient in the liquid, and they would not generally be considered low-power switching methods.